1. Field of the Invention
This invention relates to DC-to-DC power converters, and more particularly to resonant power converters.
2. Description of the Prior Art
Various DC-to-DC power converters are available for transforming an input DC voltage of one magnitude to an output DC voltage with a different magnitude. Two conventional converter topologies are referred to as the flyback and the forward converters. They are discussed, for example, in a text by George Chryssis, "High-Frequency Switching Power Supplies: Theory and Design", McGraw-Hill Book Co., 1984, pages 11-13.
With a flyback converter, a switch is connected in series with the input winding of a transformer. The switch is alternately turned on and off, producing a pulsing in the secondary winding which is fed through a diode to charge an output capacitor. When the primary current is switched off, the current in the secondary tends to surge. The rate of change of both the primary and secondary currents are very high, leading to electromagnetic interference and radio frequency interference. Complex filters are required to suppress the interference, thereby increasing the complexity and cost of the system and reducing its efficiency.
In the forward converter design an inductor is added to the secondary circuit to reduce the absolute current magnitude in the secondary, while a second diode in the secondary circuit closes a circuit between the output capacitor and inductor when the input switch is off. This design uses a high input current, which is stressful for the switching transistor in the primary circuit. The output diode is stressed by large voltage and current swings, requiring a snubber circuit which adds to the cost and complexity of the system and is an interference source. The large rates of current change in the transformer windings and in the inductor produce electromagnetic and radio frequency interference, which again require complex filters to remove.
Many of the problems associated with flyback and forward converter designs are resolved by the more recent "resonant" converter, which is exemplified in U.S. Pat. No. 4,415,959 to Vinciarelli. In this type of device, the most relevant embodiment of which is shown in FIG. 4 of the patent, a relatively large inductor acts as a current sink in the secondary circuit. A capacitor in the secondary circuit cooperates with the leakage inductance of the transformer to establish an effective LC circuit; this defines a characteristic time scale for the rise and fall of current from the DC voltage source. A switch device in the primary circuit can thus be switched on and off at essentially zero current, thereby overcoming the problems in both the flyback and forward converters associated with switching under high current levels. Following each cycle the energy stored in the capacitor is released by the current sink. After the capacitor has been discharged, the sink current is carried by a diode connected in parallel with the capacitor.
While this type of resonant converter solves the interference problems associated with the flyback and forward converters, it itself has certain limitations. Its output power is modulated by operating the converter over a very wide frequency range, thus forcing the converter to operate at very high frequencies at full power if a wide dynamic range of minimum to maximum output power is required. Attempts to operate the power supply at low frequency for full output power preclude operation at low power levels. Additionally, peak currents in the switching elements increase with increasing input voltage. When the input voltage is at its maximum design rank, peak current stresses on the switching elements become severe. Moreover, the design does not lend itself well to multiple outputs.